Coaxial cables are nowadays in widespread use for numerous applications, and in particular for transmitting signals at radio frequency. For example, such cables are known for conveying television signals or telephone signals for various generations of cellular telephones.
A coaxial cable comprises a central conductor surrounded by a dielectric and by a peripheral conductor which is generally protected by a polymer sheath. In many cases, the dielectric is constituted by an expanded and extruded polymer, and the peripheral conductor is made of copper or of aluminum. In numerous cases, the peripheral conductor is corrugated so as to give the coaxial cable flexibility that is compatible with conditions during installation while nevertheless guaranteeing the best possible transmission qualities.
It is well known that these transmission qualities depend both on the intrinsic qualities of the inner and peripheral conductors and of the dielectric, and also on compliance with various nominal geometrical values for said conductors, these values being characterized by the outside diameter of the central conductor, by the inside diameter of the peripheral conductor or an equivalent diameter if it is corrugated, and finally by the regularity of these intrinsic characteristics or these nominal values along the axis of the cable. The method of fabrication is thus a major contributor not only to the economic aspects of the finished cable both in raw costs and in costs related to fabrication efficiency, but also in the ability to make the cable at high speeds while maintaining good quality.
Although known methods do indeed enable high quality products to be made that are widely available, they nevertheless suffer from certain drawbacks. Thus, the methods in the most widespread use are based on taking the already-fabricated central conductor surrounded by its dielectric from a supply reel, and then inserting this cable element into the cylindrical peripheral conductor which is being pulled by a “caterpuller” and which is then corrugated prior to being wound onto a take-up reel. On leaving the upstream reel, the central conductor element surrounded in its dielectric passes through a jumping roller system serving to regulate the speed at which it is unwound from the reel. On leaving the corrugator, the cable element including the corrugated peripheral conductor passes through another jumping roller system serving likewise to regulate the speed of rotation of the take-up reel.
In that method, the reference for winding speed is the speed of the caterpuller for pulling the peripheral conductor. As a result, and in spite of regulation at the supply reel and at the take-up reel, the corrugating head is subjected to a force that varies continuously both in magnitude and in direction because of the continuous variations in the speed of the supply reel, which corresponds to the quantity of the intermediate product varying per unit time, and to continual variations in the speed of the take-up reel, which in turn corresponds to the quantity of finished product varying per unit time.
As a result, that method implies measuring or estimating the axial force acting on the corrugating head and continuously returning said head to its equilibrium position by modifying the speed of rotation of the tooling used for making the corrugations. Such modification in speed itself gives rise to modifications in tension over the entire line and thus acts via the above-mentioned jumping roller systems to modify the delivery and take-up speeds of the above-mentioned reels.
This situation can be summarized by observing that in such a method the peripheral conductor is supplied to the corrugator at a given speed which is the speed that acts as the speed reference for the fabrication line. The quantity of conductor thus supplied is equal to the quantity of conductor consumed by the corrugator, with this quantity thus being a function of the reference speed of the caterpuller and of the diameter of the soldered conductor tube prior to penetrating into the corrugator. This same quantity of conductor that is consumed is naturally a function of corrugation parameters, and in particular the diameter of the corrugation ridge, of the corrugation furrow, and of the shape of the corrugation. The central conductor carrying its dielectric is entrained by the corrugating operation which is performed by applying compression to the dielectric and tension is needed to tighten the central conductor element with its dielectric.
The use of such a method thus requires both a speed to be defined for the peripheral conductor (which is in the form of a tube prior to being corrugated), and a diameter to be defined for said peripheral conductor, and it is then necessary to compute an approximate speed of rotation for the corrugating head. During startup tests, the various tensions are adjusted in order to obtain the desired dimensioning, in particular the tension for unwinding the supply reel and the tension for accumulating cable on the take-up reel.
Under those circumstances it will be understood that changing a single parameter requires all of the other parameters to be changed in order to conserve the desired dimensioning, which means firstly that operation is too complex for excellent efficiency to be likely, and secondly that there are numerous risks of malfunction which lead either to poor quality or else to additional production going to scrap.
Finally, in such a method, given the variation in the speed of the supply reel and thus in the tension induced for the cable element that is inserted into the peripheral cylindrical conductor (i.e. the element comprising the central conductor with its dielectric), it can be assumed that the slip of the peripheral conductor over the dielectric is never constant. This is particularly important when the incident cable element comprising the central conductor carrying its dielectric passes through only one jumping roller system on leaving the initial reel since it then tends to retain its initial curvature that it had on the reel. Unfortunately, this curvature is not uniform and changes as the reel unwinds, which means that this cable element is naturally of unstable position at the time it is inserted into the peripheral conductor. This means that parameters need to be adjusted in order to produce a compliant cable. Furthermore, irregular distribution of the dielectric on the upstream reel is transferred directly to the corrugator in terms of varying tension that gives rise to variation in the shape of the corrugations.
Existing methods are thus based on continuously regulating a system comprising a plurality of parameters around an optimum operating point which can itself be obtained only after making progressive adjustments. (Some existing methods do not even include the operating flexibility provided by the corrugating head having a floating mount which can be used to regulate the speed of rotation of the corrugating head.) In all cases, existing methods are thus methods of making high quality cables but at the cost of great difficulty in terms of controlling and governing the method, and requiring vigilance that is poorly compatible with certain modern production requirements, and speeds that are very limited. All of these points inevitably have consequences, even if relatively small, on the good quantity of a product such as a coaxial cable, even though its performance is specifically determined, in part, by the need for a very high degree of geometrical uniformity in its component elements. Finally, it also follows that efficiency is fairly bad due either to time required for adjustment purposes or to lengthy readjustments requiring operations that are difficult, or else to reject rates that are rather high due in particular to drift in performance.